Infra-red sensitized photographic materials are known for quite some time in photographic technology. One of the well-known early applications was and still is in aerial photography. The most familiar classes of infra-red sensitizing substances are long-chain cyanine dyes. Suitable infra-red sensitizing dyes are disclosed in e.g. U.S. Pat. Nos. 2,095,854, 2,095,856, 2,955,939, 3,482,978, 3,552,974, 3,573,921, 3,582,344, 3,623,881 and 3,695,888.
In recent years new techniques of image recording have emerged wherein the exposure source of the recording apparatus is a laser unit. So, for example, in the pre-press field of phototype- and image-setting, recorders are marketed which employ Ar ion lasers, He-Ne lasers or solid state semiconductor lasers, also called laserdiodes, as exposure source. This latter type of laser, the laserdiode, shows some advantages compared to other laser types such as low cost price, small size, long life time and no need for an acoustic-optical modulator. Generally the emission wavelength of these semiconductor laser beams is longer than 700 nm and mostly longer than 750 nm. So photographic materials appropriate for exposure by devices employing such laserdiodes must be sensitized for the near infra-red region of the radiation spectrum. Suitable photographic materials to be used with semiconductor laser device are disclosed in Japanese Unexamined Patent Publication (Kokai) No. 61752/85 and U.S. Pat. No. 4,784,933. Commercial infra-red sensitized film and paper were announced by EASTMAN KODAK Co. in "Proceedings Lasers in Graphics, Electronic Publishing in the '80's, Vol 2 (September 1985) p. 508. Other manufacturers include AGFA-GEVAERT N.V. and FUJI PHOTO Ltd. An example of an image-setter using a laserdiode exposure is the CG 9400 apparatus marketed by AGFA COMPUGRAPHIC, a division of AGFA CORPORATION.
Another rather recent application of lasers as exposure units is situated in the field of radiographic monitoring photography where a hard copy of radiographic information has to be produced. The laser imager or recorder is an optical/electronic/machanical device which forms an alternative for the more conventional video imager. In a video imager one image is captured on a CRT and photographed. In a laser imager the digital image information required for one film sheet is put, via an interface, into a memory store. At the moment of recording, the full information stored in the memory is used to modulate the laser beam via an acoustic-optical modulator (except in the case of laserdiodes) in terms of brightness, grey levels etc. The radiographic information, originating e.g. from a CT-scan or a NMR-scan, is written line per line on the photographic output material e.g. in a horizontal plane while the photographic material itself is moved in a vertical direction. Although the video imager shows some advantages such as faster data transfer resulting in a shorter exposure time, less complicated interfacing and less susceptibily to dust and vibration, the laser imager shows the far better overall picture quality thanks to the higher resolution. Most laser recorders use HeNe lasers which require red sensitized output materials, e.g. the laser imager marketed by AGFA MATRIX, a division of AGFA CORPORATION. However the widely used laser recorder marketed by MINNESOTA MINING AND MANUFACTURING Co operates by means of a laserdiode emitting at 820 nm. So again for this type of device a hard copy medium sensitized to the near infra-red is needed.
A permanent problem in the field of imaging by laserdiodes is the sensitivity level of the infra-red sensitized photographic material. A first reason for that is made up by the low energy output of the laserdiode which is in the order of a few milli-Watts. A second problem consists in the usually poor storage quality of emulsions sensitized with long-chain cyanine dyes unless considerable amounts of stabilizers are used; however it is generally known that such a strong stabilization tends to reduce the original sensitivity level. An at least partial remedy for the sensitivity problems in infra-red photographic materials was found in the use of so-called "supersensitizers". Suitable supersensitizers are disclosed e.g. in U.S. Pat. No. 3,695,888, in U.S. Pat. No. 4,603,104 and in Research Disclosure Item 28952, published in May 1988. Although the sensitivity of infra-red materials is greatly improved by the use of supersensitizers it can still be insufficient; so there is a permanent need for new ways of enhancing this sensitivity.
It is an object of the present invention to provide a new way of improving the speed of infra-red sensitized photographic emulsions.
It is another object of the present invention to provide hard copy photographic materials, coated on a transparent base, for laserdiode recording, which show the desired sensitivity.